Feedback amplifier



June 9, 1942.

E. H. PERKINS. FEEDBACK AMPLIFIER Filed April 20, 1940l /sa/qf//Q/ f4 /52 //9o l, r 'l FG. v

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100.0 00 200.000 MEQUENCV IN CVdES PER SECONQ A TTORNE'Y lwhich {a} equals'or exceeds unity. phase angle of the loop l'propagation a, the symbols u and having the significance indicated in Patented June 9|, 1942 I UNITED 'STATES PAT-ENT oFFlcE I FEEDBAZZKSILIFIER l l I Edwin H. Perkins, Basking Ridge, N.

to Bell Telephone Laboratories,

J., assignor Incorporated,

` New York, N. Y., a corporation of New York Application Apta 2o, 1940, serial No. 330,671 1 7 claims. (ci. 17a-'171) This invention relates to wave translation and especially to systems involving feedback ampli.

fiers.

Objects of the invention are to control feedback and oscillations in such systems, to increase the width of transmitted frequency band obtainable in feedback amplifiers and the lop gain `obtainable over the transmitted vfrequency band,

to facilitate starting o'f conditionally stable feedback-amplifiers or stable negative feedback amplifiers having loop phase equal to zero at a frequency for which the' steady-state loop gain exceeds zero decibels, and to reduce deleterious ef- Bode Patent 2,123,178, July 12, 1938) is designed to constitute a system that has been' referred to as a completely stable system, in order to provide definite and preassigned gain and phase margins against singing. Provision of such margins ordinarily involves constructing the feedback loop or a loop so that the loop phase shift is not allowed to become zero for any frequency for which the loop gain equals or exceeds zero decibels, orin other words so that 1l is not allowed to'become zero for any frequency for` I is the fects upon amplifiers of such character caused by bie with conditional stabinty than with oom- -plete stability. However, special problems arise in connection with getting a conditionally stable amplifier into operation and maintaining this stable operation under varying operating conditions that may aect the loop gain, as for example varying loador varying power .supply voltages.

v For instance, in starting a conditionally stable negative feedback vacuum tubeamplier, that is, when rst turning on the tubes or conditioning them for ampliiication, there may be a period in which the amplifier is unstable and starts singing (i. e. oscillating), before the temperatures cf the cathodes of the tubes have reached their final values (i. e. their normal operating values) or while the transconductances ofthe tubes are building up or increasing toward their the above-mentioned paper and. patents.) A stal bilized feedback amplifier having what has been because of decrease in transconductance of vacuum tubes in the circuit with time, due to the tubes .becoming agedthrough use.

In stabilized feedback amplifiers, ordinarily a rence, and the relay may rapidly remove the loss,

normal values after application of theenergizing potentials for-the tubes; also, because of some abnormal operating condition, as forV example amplifier overload or -temporary"'ieduction of power supply voltage,- the tube transconductances and consequently lopl may decrease, to'

such abnormal values that the a'mpliiier becomes unstable and starts singing. In either case the singing may persist. That is, the singing so occasioned may continue after the amplifier has been turned on or after the overload'or other abnormal operating condition has ceased.

A specific ,feature of the invention relates to the provision of means for automatically stopvping such oscillations due to subnormal tube -transconductance, upon their occurrence. For

example, a relay responsive to changes of unidirectional space current. of one of the tubes produced .by the `building up of the oscillations Vand by their decay may introduce suiiicient loss or decrease of transmission eiiiciency in the feedback loop'to vstop the oscillations due to subnormal ltube transconductance, upon their occurafter the tube transconductance has become sufliciently high to render-the amplifier stable greater operating frequency range or a greater l amount ofnegative feedback, or both, are attainwith the loss removed, the removal of the loss being accomplished with sufficient. rapidity to prevent -oscillations from building up as the loop gain increases due tothe loss removal. For instance, the relay may operate, upon the building up of oscillations, to cause a short circuit to be established across the feedback loop at some point in the loop so that the oscillations decay or cease, and the relay may operate, upon decay or cessation of ,the oscillations and consequent return of the unidirectional space current to approximately normal value and increase of tube transconductance to approximately normal operating value, to cause removal of the short circuit sufdciently rapidly to prevent oscillations from building up to the removal of the short circuit.

Other objects and features of the invention will be apparent-from the following description and claims.

In the drawing:

Fig. 1 is a circuit diagram of an amplifier embodying a form ofk the invention;

Fig. 2 shows gain-frequency characteristics of the amplifier; and

Fig. 3 shows the ri-characteristic of the ampiier or polar plot ofthe loop propagation incl, Lr over a frequency range extending from far below the operating range to far above the operating range.

Fig. l illustrates application of the invention to a feedback amplifier -of the type employing repetition of feedback. (a type disclosed, for example, in H. S.Blacks above-mentioned Patent 2,102,671'and in his Patent 2,209,955, granted August 6, 1940, on copending application Serial No. 114,390, filed December 5, 1936, for Wave translation systems), with the repetition taking place through an outer feedback path f from an output hybrid coil 1 'to an input hybrid coil 5. The amplifier comprises tandem connected tubes |5|, |52 and |53, and has an output bridge |54 joining tube |53 to the hybrid coil 1 and to an inner feedback connection |55. This connection |55 applies the voltage from the bridge |54 across a feedback impedance in series with the secondary winding of the input hybrid coil with respect to grid-cathode impedance of tube |5|.

The four -ratio arms of the bridge |54 are the anode-cathode impedance of tube |53, resistances |55 and |51, and impedance 58. The arm |58 comprises a resistance |59 in parallel with a capacity |50 (of the order of 50 micro-microfarads, for example) for controlling the slope of the attenuation-frequency characteristic of the inner feedback path, to control the phase shift above the transmitted band. The bridge |50 may be unbalanced, if desired, with resulting advantages brought out, for example, in H. S. Black Patent 2,131,365, September 27, 1938.

The feedback connection |55 includes a stopping condenser |9| for preventing passage of direct current through this connection. The feedback impedance in series with the secondary winding of hybrid coil 5 and included in the feedback diagonal of bridge |54 comprises twoseries connected elements; first, a. feedback resistance |82; and second, a feedback resistor |63 in parallel with an inductance |64, ing to cancel the phase shift introduced by the blocking condenser |6| in the inner feedback path at .low frequencies. A grid bias resistor |55 and by-pass condenser |88 are shown in the grid circuit of tube |5|. Tubes |5'I and |52 may be Western Electric Company type 310A tubes and tube |53 a Western Electric Company type 311A tube, for example. A 24-volt A battery or other suitable direct current source A with its positive pole grounded and its negative pole con? nected to the -A terminal supplies heating current to the filaments of tubes |5I, |52 and |53 in series. A 13D-volt B battery cr other suitable direct current source B connected from ground to the -l- B terminal supplies space current for tubes and |52, and these A and B sources-in series supply space current for tube |53.

as the loop gain increases due this inductance serv' ing plate current of rent between the cathode of tube A grid bias resistor |10 by-passed by a condenser |1| supplies grid bias voltages for tube |52. Grid bias voltage for tube |53 is supplied through grid filter resistance |98 by grid biasing battery |12 and any direct current drop in inductance |13 which is in series with battery |12 and the direct current path from the cathode to the grid. This inductance serves, with condenser |15 which is a by-pass from the cathode of tube |53 to ground, as vfilter to prevent the alternattube |53 from ,flowing through the A battery. Condenser |16 and resistance |98 filter the biasing voltage for the grid of tube |53; and the condenser |15 provides a by-pass aroundfbattery |12, resistance |98 and the A battery in series. A choke |11 and condenser |18 serveas a filter isolating the B battery with respect to alternating current; and condensers |15 and |18 bypass alternatingcurl|53 and the bridge arm |55. Resistance-capacity filters for the direct current grid biasing circuits of tubes |5| and |52 comprise resistors |80, |8| and |82 and capacities |83, |84, |85 and |86.

Fig. 2 shows the gain-frequency characteristics A and \B of thelamplier of Fig. l between terminals T1 and T2 for operation without feedback and with feedback respectively, the particular amplifier for which these` characteristics were measured being amplifier for a twelve-channel cable carrier telephone system.

In this amplifier, tubes,|5| and |52 are coupled by an interstage coupling network |80 comprising an air core or magnetic core transformer |81, a parallel-resonant circuit |88, a resistance |83,

and stopping condensers or by-pass condensers f with the interstage shunt capacity,

` stage network |90 high over the I9! and |92. The transformer has a primary winding |93 and a secondary winding |94 which may have, for` example, a 1:1, turns ratio, these windings being connected at one end by the stopping condenser l9i, which connects the plate of tube |5| to the grid of tube |52, and at the other end by the condenser |92. Each of these condensers |9| and |92 may have a capacity of the order of .l microfarad, for example. Resistor |89 is a grid leak having a resistance, for example, of the order of 60,000 ohms, providing a, direct current path for biasing the grid of tube |52 without producing undue shunting effect upon-the resonant circuit |88. This resonant circuit |88 is shown as a coil |95 and a capacity |95 which may be the self-capacity of the coil or may include a condenser separate from the coil. The interstage network |90 becomes parallel resonant self-capacity of the transformer, at a frequency in the neighborhood of the -kilocycle peak P1 in curve A; and the resonant circuit |88 resonates at a' frequency in the neighborhood of the 15-kilocycle peak P2 in curve A. e transformer |81 renders the coupling impedance of the interupper portion of the utilized frequency range and the resonant circuit |88 renders the impedance of the interstage network |90 high over the lower portion of the utilized frequency range, the two cooperating tomaintain the impedance of the coupling circuit high over the whole of the utilized frequency range, to the end that the amplification of the amplifier without feedback may be high and n ot unduly variable with frequency over the utilized frequency range.

Tubes |52and |53 are coupled by an interstage plate supply circuits and screen a 12- or GO-kilocycle transmitter including the,

network |90 structurally and functionally similar to the interstage network |90. The frequency ofthe peak P1 in curve -A is determined by theresonance frequencies of transformer |81 and the corresponding transformer in network |90', and these resonance frequencies may differ somewhat. -Similarly. the frequency of the peak Pz in curve B is determined by the resonance frequencies of circuit |88 and the corresponding resonant circuit in network |90', and these two resonance frequencies may be staggered.

In the feedback path f is a deviation equalizer 200 comprising a parallel-connected resistance 20|, inductance 202, and a capacity 203, shunted across the path f. This deviation equalizer in theY outer feedback path renders the over-all gainf requency characteristic of the amplifier-practically exactly fiat over the utilized frequency range. For'example, in the 12kilocycle to 60- kilocycle transmitting amplifier for the twelvechannel cable carrier telephone system, this equalizer rendered the gain fiat within about .01 decibel over the transmission band. Where it is desirable to be able to reduce the gain of an amplifier with feedback to as 10W a value as possible andan equalizer is to be used in the feedback path, it is advantageous to have the a loop include the input and output transformers in the general manner of Fig. 1, for example.

In the feedback path f is a network' 205, comprising resistances 2.06, 201 and 208, and the capacity 2| which functions to increase the transmission loss in path f in the transmitted band of the amplifier, thus increasing the amplifier gain over that obtained with no transmission loss in the feedback path. The condenser 2|| in parallel with resistance 201 is used to improve the phase shift at high frequencies and to decrease the loss of network 205 atfrequencies above the utilized frequencyA range.

A transformer 2|5 which may have a -turns ratio of 1:1, or any desired ratio, is shown in the feedback path f. The transformer improves the y longitudinal balance of the system, and enables the circuits connected to its two windings to be one symmetrical andthe other unsymmetrical (unbalanced-to-ground) Fig. 3 shows the steady-state api-characteristic of the amplifier (assuming the lines or terminating impedances attached at terminals T1 Aand T2 have the normal values of 600 ohms and 135 ohms respectively). This is the steady-state characteristic obtained, with both the inner and outer feedbackpaths effective, when the point at which the circuit is broken for measurement of i loop propagation is a point of the )iL-circuit or forwardly transmitting amplifying path common to both the inner and outer feedback paths (in loop gain exceeding zero decibels, and with further frequency increase there is a cross-back as the characteristic passes from the fourth quadrant back into the first quadrant at frequency F2l with lat still exceeding unity. With frequency decrease below the frequency in the operi ating frequency range at which I =180, the

characteristic has a cross-over point at frequency f1 where it passes from the fourth quadrant into the flrstquadrant at a value of loop gain exceeding zero decibels, and with further frequency decrease the characteristic has a crossback as it passes from the first quadrant back into the fourth quadrant at frequency f2 with the loop gain still exceeding zero decibels:

(The amplifier of Fig. 1 as so far described may be, for example, that of Fig. 15 of th above-mentioned Patent 2,209,955.)

In a conditionally stable feedback amplifier, the problem of causing the amplifier to be stable when its vacuum tubes or amplifying elements have reached their steady-state operating conditions after application of their energizing potentials, for instance, application of filament heating potentials v byswitch S in Fig. 1 with lthe energizing cir/cuits otherwise comple-ted, and 'alsoy the problem of causing the amplifier to be stable after an overload or other abnormal operating condition decreasing the loop gain suiiiciently to render theamplifier unstable has been removed, arises if` the steady-state ri-characteristic loops from the first quadrant into the fourth quadrant at high frequencies in the fashion indicated in Fig. 3, for example, or loops from the fourth quadrant into the first quadrant at low frequencies in the fashion indicated in Fig. 3, for example. This is because, in orderfor [al to'reach its steady-state value, a must pass through a region of values which cause the ,ic-characteristic to enclose-the point 1, 0, or in other words cause the amplier to be unstable.

A solution for these problemsllies in circuit additions shown in Fig. 1 as relays 30| and 302 and resistance pad or impedance correcting netf work 303. Relay 30| has its winding connected 'in the unidirectional space current circuit of tube |53 and is adapted for marginal operation by. unidirectional space current of that tube for causing its contact 306 to connect winding 3| 0 of relay 302 across the battery B. Relay 302 is adapted to operate rapidly and release slowly. Contact 3|| of relay 302 is for establishing a short circuit across the outer feedback path f of the amplifier. .Contact 3|2 of relay 302 is for vestablishing a short circuit across-the amplifier input terminals T to which is attached the in- 1 coming line or circuit.

contradistinction to a. point of'either feedback ,This `,nfl-characteristic shows that the loop phase shift is zero for four values of loop gain exceeding zero` decibels. In other words, the

characteristic crosses the real axis Ifour times. with loop gain exceeding zero decibels, or the loop phase shift crosses through zero value at four frequencies for which la] exceeds unity.A

Thus it may be said the characteristic has four, cross-overs or cross-over points on the real axis for values of IMSI exceeding unity. l' With frequencyincrease above al frequency in the operating frequency range at which i =180, there is a cross-over of the characteristic at frequency f1 as the characteristic passes from the first quadrant into the fourth quadrant at a value of- The inner feedback path is such that with the `outer feedback ineffective or ii'factive (for example, short-circuited by contact 3| I) the amplifier is completely stable. That is, the amplifier can sing only when the outer feedback path |55.

is active. The normal space current of the 311B tube |53 is 30 milliamperes. When the amplifier sings, the space currentof this tube builds up to 4a. value of approximately 55 milliamperes, which may be considered 'its normal singing value. This building up happens because the action of feedback is such that with full load output the space current seeks a value such that the voltage and currentlimitations of the tube will'be realized to-"' gether. This circuit is so adjusted that the no load space current is somewhat less than the full load value. When singing occurs, because of the fact that no limiting is incorporated into the circuit, the circuit will vsing violently enough to`operate thetube at full load output. Under normal service conditions', this yfull load output condition is not obtained except for instantaneous peaks, and the relay 30| will not respond for currents of such short duration. The relay 30| is adjusted to close its contact when a current of about 50 milliamperes flows in its winding 305 (so it will always receive sufficient current for operating it when oscillations build up in the amplifier to a steady-state amplitude), and this relay is adjusted to release its contact when the current decreases to about 35 milliamperes. The contacts of relay 302 are so associated that, in opening, they open the short circuit across the feedback path` f, first, and the short circuit across The operation of starting the amplifier or turning on the tubes to`condition them for amplifying will now be described. During this operation the amplifier may be disconnected froml the incoming and outgoing lines, if desired.

.When switch S is operated to its closed position the two 310A tubes I5| and |32 come up to normal transconductance in about a minute, but in that time the transconductance of the 311A tube |33 has reached only about |00, o'rsuch a fraction of its normal transconductance as to make the la-characteristic of the amplifier enclose the point 1,/0 and therefore make the amplifier start and the space current of tube |53 decreases to its normal operating value of 30 milliamperes, so relay 30| releases its contact, causing relay 302 to open the short circuit across the feedback path f and bring the amplifier to its normal operating condition. Relay 302 is made a slow-releasing relay, since the increase of ht! to its normal operating value may be slower than the decrease of the space current to its normal operating value and so if the relay releases to'o quickly the singing condition may not be cleared before the may lee-135 ohms, for exemple. 1f the empnner is to be started'as described above with the amplier not connected in circuit, it may be necessary to consider 'the effect. of an infinite load impedance (i. e., an open-circuit condition at terminals Ta) in the design of the amplifier to insure against possibility of singing. However,l if bridge |54 is in passive balance and the impedance of hybrid coil 1 facing bridge |54 balances impedance |0,'so that the load is conjugate to the feedback path f, then the load impedance does not affect stability of the amplifier against vthe input terminals T afterward. That is, contact 3| I opens before contact 3|2.

singing. f

The operation of the amplifier under overload will now be described. As indicated by the acharacteristic in Fig. 3, if the amplifier is so overloaded as to cause Inl to. decrease by about l0 decibels or more; a singing condition exists; and the singing persists afterthe -load signal falls below the overload value (or, in other words, the amplifier locks into the singing condition) unless the amplifier is controlled. It is controlled as* 30| releases its contact upon ,the restoration of the space current of tube |53 to normal operating value. Relay 302 is thereby deenergized. It

' opens its contact 3| I thus bringing the amplifier to its normal operating condition,`and thereafter it opens vits contact-3H thus removing the short circuit across terminals T.

1f the overload has vanished when the cycle of 1 operation is thus completed, the operation of the amplifier is normal. 'Ihe complete cycle may take less than a second of4 time when caused by a peak overload of-short duration. An instantaneous peak such as exists in speech will not cause the relay 30| to'operate, because the full load space current it causes exists for too short feedback path is again rendered operative. A

hunting condition might thus be established if the relay were not a slow-release relay.

In starting-the amplifier as Just described, with the amplifier not connected in the transmission line or circuit, the short-circuiting of the amplifier input terminals T is not necessary. However, when the feedback path f is operative, the amplifier may sing if the impedance viewed from the input terminals T1 of the hybrid coil 5 when looking toward terminals T differs too greatly from the normal value (which may be 600 ohms). For example, -ii' the impedance so viewed from terminals T1 is zero or infinity, (the values of a given by Fig. 3 do not obtain and) such improper value of impedance may cause the amplifier to sing. 'Ihe impedance correcting fnetwork 303 insures that, even with terminals T open-circuited or short-circuited, the impedance Y so viewed nfrom terminals T1 will notdifier from a time to operate the relay.

If, on the other hand, a continued overload is present, the cycle of operation is repeated until the overload is removed. Repeated operation of the relay cycle can be caused to bring up a circuit alarm if desired for example, by having relay 30I also operate a counting relay, not shown, which will introduce an alarm after any desired number of cycles of operation of relay 30|. Such an alarm is desirable because if the amplifier were permitted to continuously repeat this relaycycle, it would indicate a circuit trouble and also the continuing cycle of operation would constitute a circuit interruption.

The sequence in which the contacts of relay 302 open insures that the short circuit across the feedback path ,f will always be removed before the short circuit across the terminals T is removed. Otherwise the normal full load input would effectively be too high; for with the short circuit across ,f removed, the feedback through path f may reduce the amplifier gain 15 decibels, for example, and with a short circuit established acrossthe feedback path f the amplifier which,

without the short circuit across f, may have a normal gain of 55 decibels, for example, has a gain of 70 decibels.

In the operation of the amplifier upon occurrence of overload, when terminals T are short- .circuited, a singing condition might exist because Vof `this short circuit if network 303 were not present. Then as soon as the feedback path overcome by the use of thev network 303 between the short circuit at terminals T and the terminals T1. As indicated above, the network presents thel proper impedance to terminals T1, notwithstanding the short. circuit across the terminals T to prevent inputsignals from entering the amplifier. Thev transmission loss in the network 303 may be lof the order of 3 decibels, for example.l l

When the amplifier is started while it is connected in a transmission circuit, the short circuit across terminals T is advantageous in preventing any shock excitation entering the amplifier from the input line or circuit that might tend to set the amplifier singing when the short circuit across the feedback path f is first opened.

Although the slow-release'characteristic of rethe short circuit across the outer feedback path and start the amplifier singing; for decreasing lppl decreasesthe shock.

If desired, filters (not shown) for suppressing waves of singing frequency may be used to connect terminals T and T2. of the amplifier to its attached incoming and outgoing circuits, for

excluding the oscillations of the singing fre-l quencies from thosecircuits. Either filter, or

both, may be omitted, depending upon the i specific application of the amplifier.-

lay 302 is advantageous as indicated above, the

removal of the short Vcircuit across the feedback path by contact 3H, once started, should be effected rapidly enough to increase llfl through its unstable region of values in a time so short that oscillations cannot build up to amplitudes that would limit causing grid current flow, the space current of tube I to operate relay l Impedance levels ordinarily are lower in the feedback 'path than in the llt-circuit. Consequently, locating the short circuit for the loop path in the feedback path rather than in the p-circuit is advantageous in minimizing deleterior that would -cause ous effects of' capacity introduced across the loop path by the short-clrcuiting means. However, the low impedance of the feedback path tends to increase diiculty of effectively shortcircuiting the feedback voltage. Theamplifler using repetition of the feedback process, with the inner and outer feedback paths, is advantageous in facilitating effective short-circuiting actionl of the means for establishing the short circuit across the outer feedback path; for example, the total reduction of the insertion gain of the amplier Vto be effected by feedback may be 55 decibels, of which 40 decibels may be obtained bythe inner feedback path, leaving only 15 decibels to be effected by the outer feedback path, so the feedback voltage to be short-cir- 53 to frise sufficiently amplifier of Fig. 1, for lul, for example, by

Also, if desired, a voltage limiter (not shown) may be connected between the incoming line and terminals T, to reduce danger of overload causingthe amplifier tostart singing. It may be of any suitable type, las for example, a negative feedback amplifier' with its gain-load characteristic having a shart cut-off, -as in the' case of Fig. 10 of the above-mentioned paper of H. S. Black, the overload point or cut-off value of load ybeing adjusted, for example, to a load value sufficiently low to insure that the input to terminals T cannot reach ka value so large as to cause the ,acharacteristic of Fig. 3 to shrink in such man ner as to -pas's through or enclose the point L@ In a conditionally stable amplifier, such as the example, if l/fl. is to be varied, for instance, by varying the attenuation o f network 205 to vary the insertion gain of the amplier by changing the -circuit loss and thus the amount of feedback, the decibel spread in Fig. 3 between the point ,u=1,/0 and th'e tact 3H, need not in all cases have zeroimpedance. It should, when closed, render the] amplifier completely stable. .In other words, it should be capable of changing the amplifier from` a conditionally stable state -to a completely stable state.

In the claims stable is used in the sense that the response, to a small impressed force which dies out in the course `of time, also dies out. A well-known criterion for this stability is Nyquists rule. One way of stating the rule gives as the -necessary and suflicientcondition for stability y the requirement that the graph of the imaginary part ofthe loop propagation o'r transfer factor cuited across the feedback path is relatively.

small and is therefore relativelyv easy to shortcircuit effectively. In other words it is advantageous to employ an amplifier, of the type shown in Fig. 1, using a repetition of the feedback process, with inner and outer feedback paths, and with the amplifier completely or unconditionally stable when'only. the inner feedback path is active, because under this condition only the portion of the lpl gain represented by the increase 1n a due to the looped-over portion of the ,i-

vcharacteristic need exist in the outer feedback loop, and this reduces the approach to an absolute short circuit necessary to accomplish the -desired result. As just indicated, in .the case of the amplifier'v of Fig. 1, the total llfl was about 55'decibels but that effective in the outer path was only about `15 decibels. The amplifier, using repetition of the feedback process, `with a considerable portion of the total vnegative feedback, obtained by the inner feedback path, is also advantageous in reducing shock around the feedback loop plotted against the real part for all real ,values of frequency avoid encircling the point 1,0.

In the expression conditionally stable ap-l pearing in the claims, "stable has the signicance just indicated, and conditionally signifies that the stability is conditional in the sense above indicated.

What is claimed is:

1. A Wavev translating system comprising an electric space discharge wave amplifying device, means forming with said device 'adapted to produce feedback with the loop gain exceeding zero decibels lfor a frequency at which the loop phase shift is zero and with the graph of the imaginary part `of the loop propagation plotted against the real part for all real values of frequency avoiding encircling the point'1,0, and means responsive'to a direct current component of space current. of said device for conexcitation that might occur due to removal of 75 trolling the loop propagation. 2. An amplifier plifying device, a feedback path therefor forming therewith a feedback loop for producing feeda feedback loop comprising a vacuum tube am stable, and means responsive to change in steady space current of said amplifying device for establishing a high admittance across the feedback loop. Y

3. An amplifier comprising a vacuum tube amplifying device, a feedback path therefor forming therewith a feedback loop for producing feedback that renders the a-m-plifier conditionally stable, a low impedance path for connection across said feedback loop to -form a low impedance shunt across said feedback loop, and means controlled by a direct current component back that renders the amplifier conditionally the loop phase shift changes sign at zero degrees and 'whose loop propagation has the graph of its imaginary part plotted against its real part for all real values of frequency avoid encircling the point 1,0, a circuit for supplying unidirectional space .current to said amplifying device, and means for preventing establishment of a steadystate sing. around the feedback loop upon energization 4of said amplifying device to condition said device for amplifying, comprising relay apparatus including contacts for establishing a low impedance shunt across said feedback loop, a reof space current of saidl amplifier for controlling connection of said low impedance path across said feedback loop.

. 4. Anamplifler comprising a vacuum. tube amplifying device,'a feedback path therefor forming therewith a feedback vloop for producing feedback that renders the amplifier conditionally stable, means for forming a, low impedance shunt across said feedback loop, and means controlled by direct space current of said amplifier for removing said shunt. 1

5. An amplifier comprising a vacuum tube am` p lifying device, a feedback path therefor forming therewith a feedback loop for producing feedback that renders the amplifier conditionally stable,'

the amplifier for preventing steady-state oscillations around said loop from being produced by transient oscillations caused by said overloading.

4 7. An amplifier comprising a vacuum tube amplifying device, a` feedback path therefor forming 'therewith a feedback loop for producing feedback that renders the amplifier. conditionally stable, contacts for establishing a low impedance shunt lay winding connected in said unidirectional space current circuit of said amplifying device, and means operated by energization of said winding upon energization of said amplifying device for'controlling operation of said contacts.

10. An amplifier and an incoming circuit therefor, said amplifier comprising a vacuum tube amplifying device, a feedback path therefor forming therewith a feedback loop for producing feedback that renders the amplifier conditionally stable, and means for preventing establishment of a steady-state sing varound the feedback loop upon occurrence of overload ofthe amplifier, compris-y ing'means responsive to change in direct space current of said amplifying device for establishing a lowimpedance shunt across the feedback loop and introducing a large transmission loss -between the incoming circuit and the amplifying device.

11. An amplifier' comprising a vacuum tube amplifying device, a feedback path therefor forming therewith a feedback loop for .producing feedback that renders stable, and means responsive .to-change of steady space value causedby building up of self-sustained oscillations around the feedback loop for establishing a low impedance shunt across said feedback loop and responsive-to restoration of said unidirectional space current to its normal-value for removing said shunt.

12. An amplifier comprising a vacuum ltubev amplifying device, a feedback path therefor form'- ing therewith a dynamically stablefeedback loop having loop gain so great over a frequency band so wide that the loop phase shift is zero at a frequency for which the loop gain exceeds zero deci- 'across the loop, a plate circuit for said amplifying device, and means comprising a winding'in said plate circuit for operating said contacts upon the building up of self-sustained oscillations around the loop. Y

8. amplifier comprising a vacuum tube amplifymg device, a feedbackpath therefor forming therewith a feedback loop for producing feedback and with the graph of the imaginary part of the loop propagation plotted against the real part for all real values of frequency avoiding encircling the point 1,0 with loop gain exceeding zero decibels at -a frequency'of zero loop phase shift, a circuit for supplying unidirectional space current to said amplifying device, and means for prearound the feedback loop comprising relay apparatus including contacts for establishing a low ,venting establishment 'of a steady-state' sing ceeds zero decibels for two frequencies at which bels, a relay responsive to increase of a direct current component of space current of said amplifying device produced by building u-p of selfsustained oscillations around the loop upon energization of said amplifying device to condition said device for amplifying, and means controlled by said relay for placing a low impedance shunt across said-path.

13. An amplifier comprising a vacuum tube amplifying device, va feedback path therefor forming therewith a dynamically stable feedback loop having loop gain decrease so rapidly with change of frequency at one side of the operating frequency range of the amplifier that before the decreasing loop gain reaches zero decibels the loop phase changes sign at Zerodegrees, a switch.

thel amplifier conditionally 3 current of said amplifier from its normal decrease of said space current to its normal value upon cessation of said oscillations.

14. An amplifier comprising an electronic wave amplifying device, a feedback path from the output of saiddevice to the input of said device adapted to form with said device a dynamically stable. feedback loop having loop propagation whose graph of imaginary part versus real part for all real values of frequency avoids encircling the point 1,0 and' having loop gain so great ovei` a frequency band extending so far upwardly from a frequency at which the loop phase shift is 180 degrees that before the loop gain decreases to zero decibels with frequency -increaseabove said band the loop phase shift decreases to zero degrees, means introducing transmission loss -in said loop when the gain of said amplifying device has a subnormal value and means responsive to a steady current component of space current of said device after increase of its gain above said subnormal value for removing said loss.

l5. An amplifier comprising a vacuum tube amplifying device, a feedback path therefor forming therewith a feedback loop for producing feedback that renders the amplifier conditionally stable, a space current supply circuit for said amplifying device, said amplifying element increasing its unidirectional space current in said space current supply circuit above the normaloperating value when self-sustained oscillations build up around the feedback loop upon energization of said amplifying device, a relay operable to closed position when energized for short-circuiting said feedback pathL and a relay having a winding in said spacecurrent supply circuit for energizing said rst-mentioned relay in response to said increase of space current and for deenergizing saidl first-mentioned relay to remove said short circuit in response to decrease of said means.

space current toits normal value upon cessation of said oscillations. A v

16. An amplifier and an incoming circuit therefor, said ampliner comprising a vacuum tube amplifying device, a feedback path therefor forming therewith a feedback loop for producing feedback that renders the amplifier conditionally stable,l means for preventing establishment of a steady-state sing around the feedback loop upon occurrence of overload of the amplifier, comprising'means responsive to change of unidirectional space current of said ampliiier from its normal value for establishing a low impedance shunt across the feedback loop and introducing a large transmission loss between the incoming circuit when the gain of said device is normal and tends v to produce steady state oscillation of said amplifier when the gain of said device is subnormal andvmeans forV counteracting said tendency comprising means to control propagation of said one` of said paths andjmeans responsive to a direct current component of space current ofsaid device for controlling said propagation.l control EDwiN H. Pnnms. 

